The emission and distribution of dust of the torus of NGC 1068

TL;DR

This study uses multi-wavelength infrared and sub-mm observations to analyze the dust emission and structure of the torus in NGC 1068, revealing the importance of longer wavelengths in accurately characterizing the torus size and morphology.

Contribution

The paper demonstrates that the 30-60 μm wavelength range is crucial for probing the cold dust in the torus, providing more accurate size and morphology estimates than shorter wavelengths.

Findings

The 30-40 μm emission turn-over indicates a cold dust component at 70-100 K.

The 1-20 μm SED underestimates the torus size, which is better captured with the full 1-432 μm SED.

The 432 μm ALMA observations match the torus morphology inferred from the full SED.

Abstract

We present observations of NGC 1068 covering the $19.7-53.0$ $\mu$m wavelength range using FORCAST and HAWC+ onboard SOFIA. Using these observations, high-angular resolution infrared (IR) and sub-mm observations, we find an observational turn-over of the torus emission in the $30-40$ $\mu$m wavelength range with a characteristic temperature of $70-100$ K. This component is clearly different from the diffuse extended emission in the narrow line and star formation regions at 10-100 $\mu$m within the central 700 pc. We compute $2.2-432$ $\mu$m 2D images using the best inferred \textsc{clumpy} torus model based on several nuclear spectral energy distribution (SED) coverages. We find that when $1-20$ $\mu$m SED is used, the inferred result gives a small torus size ($<4$ pc radius) and a steep radial dust distribution. The computed torus using the $1-432$ $\mu$m SED provides comparable torus sizes, $5.1^{+0.4}_{-0.4}$ pc radius, and morphology to the recently resolved 432 $\mu$m ALMA observations. This result indicates that the $1-20$ $\mu$m wavelength range is not able to probe the full extent of the torus. The characterization of the turn-over emission of the torus using the $30-60$ $\mu$m wavelength range is sensitive to the detection of cold dust in the torus. The morphology of the dust emission in our 2D image at 432 $\mu$m is spatially coincident with the cloud distribution, while the morphology of the emission in the $1-20$ $\mu$m wavelength range shows an elongated morphology perpendicular to the cloud distribution. We find that our 2D \textsc{clumpy} torus image at 12 $\mu$m can produce comparable results to those observed using IR interferometry.